(1) Field of the Invention
The present invention relates to a radio transmission system, and more particularly, to a radio transmission system including a master station and a plurality of series connected substations connected to the master station via a radio channel.
(2) Description of the Related Art
With recent developments in mobile communications, the number of mobile terminals is rapidly increasing. To cope with an increasing number of mobile terminals and also to meet the requirements for mobile terminals, such as reduction in size and in power consumption, microcell coverages are becoming smaller and smaller, resulting in an increase in the number of base stations. As a consequence, there has been a demand for construction of short-distance, high-speed multilink communication networks for connecting a plurality of base stations and a switching station for mobile communications.
As a result of development of multimedia, such short-distance, high-speed multilink communication networks are also needed for connecting homes and hub stations. Namely, it is necessary that high-speed transmission paths should be extended to homes, and to attain this, short-distance, high-speed multilink communication networks are required.
Conventionally, in cases where wire transmission paths such as optical cable are used as multiple links of a high-speed communication network, it is almost impossible to promptly complete network installation work at low cost due to many restrictions on the occupation of roads and the use of land. For this reason, a radio transmission system is employed to implement such links.
This will be explained taking a radio transmission system for mobile communications as an example. In mobile communications, a radio transmission system is provided to connect a plurality of mobile communication base stations and a concentrator station (hub station) which connects the base stations collectively to a mobile communication switching station. Such radio transmission systems are classified into opposed type and multidirectional type. In the opposed type, the concentrator station is provided with independent antennas or radio devices arranged so as to be opposed to respective base stations. Accordingly, if the number of base stations is increased, various problems arise in respect of installation space for antennas, installation cost, frequency allocation, mutual interference, etc. In the multidirectional type, on the other hand, the concentrator station is equipped with a wide-angle directional antenna or non-directional (omnidirectional) antenna, instead of antennas opposed to respective base stations, and communications are performed by means of time-division multiplexing techniques such as TDMA. Thus, even if the number of base stations is increased, the multidirectional type is free from problems associated with the opposed type, that is, problems about installation space for antennas, frequency allocation, and mutual interference. However, the wide-angle directional antenna or the non-directional antenna is expensive, and because of scattering of transmission energy, it has low antenna gain, making it necessary to increase the transmission output correspondingly. Increasing the transmission output leads to an increase in the cost of radio devices and heat generation.
In these types of radio transmission system, the concentrator station transmits and receives radio waves directly to and from a plurality of base stations. Meanwhile, if base stations are serially connected like a chain to one another via radio channels with the base stations at opposite extreme ends connected to respective concentrator stations and a base station in the middle of the chain is designed to serve also as a relay station, then the aforementioned problems can be solved. As such radio transmission system, a conventional relay system in which a plurality of base stations are series connected to one another will be explained with reference to FIG. 10.
FIG. 10 is a block diagram showing the configuration of a conventional relay-type radio transmission system. A concentrator station 101 transmits a multiple signal at 6.3 Mbps, for example, to a base station 102 which also serves as a relay station. The 6.3 Mbps multiple signal has four slots, for example, which carry four 1.5 Mbps baseband signals to be transmitted to respective four base stations. In the base station 102, a demodulator 102a demodulates the 6.3 Mbps multiple signal to obtain four separate 1.5 Mbps baseband signals, and outputs only the baseband signal destined therefor to terminal equipment 102b. The remaining three 1.5 Mbps baseband signals are supplied to a modulator 102c, which then generates a 6.3 Mbps multiple signal while leaving one 1.5 Mbps slot vacant, and transmits the signal to a subsequent base station 103.
In the base station 103, a demodulator 103a demodulates the 6.3 Mbps multiple signal to obtain three separate 1.5 Mbps baseband signals, and outputs only the baseband signal destined therefor to terminal equipment 103b. The remaining two 1.5 Mbps baseband signals are supplied to a modulator 103c, which then generates a 6.3 Mbps multiple signal while leaving two slots vacant, and transmits the signal to a subsequent base station (not shown).
In like manner, the four base stations are successively supplied with 1.5 Mbps baseband signals respectively destined therefor.
In the conventional radio transmission system shown in FIG. 10, however, each base station must be equipped with a modulator-demodulator capable of processing a 6.3 Mbps multiple signal. In the case of the modulator-demodulator of a base station which is remote from the concentrator station 101 in particular, it is required to process a 6.3 Mbps multiple signal but in actuality handles vacant slots, which is nothing but unserviceable communication. Namely, the conventional system is associated with the problem that each base station must be equipped with a modulator-demodulator having higher capability than necessary for relaying signals. Further, a base station which is remote from the concentrator station 101 also occupies a frequency band corresponding to 6.3 Mbps, which is undesirable from the viewpoint of effective use of frequency. A problem also arises in that if a fault occurs in the modulator-demodulator of an upstream-side base station which is close to the concentrator station 101, then no signals are transmitted to the downstream-side base stations.